Thomas Piketty, professor at the Paris School of Economics, isn’t a household name, although that may change with the English-language publication of his magnificent, sweeping meditation on inequality, Capital in the Twenty-First Century. Yet his influence runs deep. It has become a commonplace to say that we are living in a second Gilded Age—or, as Piketty likes to put it, a second Belle Époque—defined by the incredible rise of the “one percent.” But it has only become a commonplace thanks to Piketty’s work. In particular, he and a few colleagues (notably Anthony Atkinson at Oxford and Emmanuel Saez at Berkeley) have pioneered statistical techniques that make it possible to track the concentration of income and wealth deep into the past—back to the early twentieth century for America and Britain, and all the way to the late eighteenth century for France.